1. Field of the Invention
The present invention relates to a motion adaptive luminance signal and color signal separating filter for separating a luminance signal (hereinafter referred to as "Y signal" or simply "Y") and a color signal (hereinafter referred to as "C signal" or simply "C") from a composite color television signal (hereinafter referred to as "V signal") in which the frequency of the C signal is multiplexed on the high frequency region of the Y signal.
The motion adaptive YC separating filter is a filter which locally judges whether a picture is a still picture or a motion picture and executes YC separation suitable to the pixel signal in that picture, at each of the locations thereof.
2. Description of the Related Art
The current NTSC signal system provides a composite signal comprising a C signal and a Y signal having its high-frequency region on which the frequency of the C signal is multiplexed. Therefore, television sets require YC separation. Imperfect YC separation causes the picture quality to deteriorate in cross color, dot crawl and so on.
With development of large-capacity digital memories, there have been proposed various types of signal processing circuits for improving the quality of picture. For example, one system which includes the use of a motion adaptive YC separation which utilizes a delay circuit having a delay time equal to or greater than the vertical scanning frequency of a television signal.
FIG. 10 is a block diagram showing one example of the conventional motion adaptive YC separating filters. In FIG. 10, the filter receives, at its input terminal 1, a V signal 101 according to the NTSC system. This signal is input to both the respective input terminals of infield YC separation circuit 4, interframe YC separating circuit 5, Y-signal motion detecting circuit 6 and C-signal motion detecting circuit 7.
In the infield YC separating circuit 4, the input signal is infield separated into a Y signal 102 and a C signal 103 through an infield filter (not shown), the Y and C signals then being applied respectively to the first inputs of Y-signal mixing circuit 9 and C-signal mixing circuit 10.
In the interframe YC separating circuit 5, the input signal is interframe separated into a Y signal 104 and a C signal 105. These Y and C signals are then supplied respectively to the second inputs of the Y-signal and C-signal mixing circuits 9 and 10.
On the other hand, a signal 106 indicative of the movement of Y signal detected by the Y-signal motion detecting circuit 6 is applied to one of the inputs of a synthesizer 8 while a signal 107 representative of the movement of C signal detected by the C-signal motion detecting circuit 7 is supplied to the other input of the synthesizer 8.
The synthesizer 8 forms a motion detection signal 108 which is input to the respective third inputs of the Y-signal and C-signal mixing circuits 9 and 10. Thus, the Y-signal motion detecting circuit 6, C-signal motion detecting circuit 7 and synthesizing circuit 8 define a motion detecting circuit 80.
The output 2 of the Y-signal mixing circuit 9 provides a motion adaptive separated Y signal 109 while the output 3 of the C-signal mixing circuit 10 provides a motion adaptive separated C signal 110.
This conventional YC separating circuit will now be described in operation.
On YC separation of V signal 101, the motion detecting circuit 80 judges whether the V signal 101 is one indicative of a still or motion picture, based on the output signal from the synthesizer 8 in which the outputs of the Y-signal and C-signal motion detecting circuits 6 and 7 are synthesized.
As shown in FIG. 11, the Y-signal motion detecting circuit 6 may comprise a one-frame delay circuit 53, a subtracter 54, a low pass filter 55 (hereinafter referred to as "LPF"), an absolute value circuit 56 and a nonlinear converting circuit 57. V signal 101, inputted to the Y-signal motion detecting circuit 6 at its input 51, is delayed by one frame at the one-frame delay circuit 53. The V signal 101 is also applied directly to the subtracter 54 and then subtracted from the one-frame delayed signal to determine one-frame difference therebetween. The one-frame difference signal is passed through the low pass filter 55 (hereinafter referred to as "LPF") and then applied to the absolute value circuit 56 whereat the absolute value thereof is determined. The determined absolute value is then converted by the nonlinear converting circuit 57 into a signal 106 indicative of the amount of movement of the low frequency component in the Y signal. This signal 106 is outputted from the output 52 of the Y-signal detecting circuit 6. The nonlinear converting circuit 57 serves to convert an absolute value into data having a magnitude which can be more easily handled by the system.
As shown in FIG. 12, the C-signal motion detecting circuit 7 may comprise a two-frame delay circuit 81, a subtracter 82, a band pass filter 83 (hereinafter referred to as "BPF"), an absolute value circuit 84 and a nonlinear converting circuit 85. V signal 101 inputted to the C-signal motion detecting circuit 7 at its input 11 is delayed by one frame at the two-frame delay circuit 81. The V signal 101 is also applied directly to the subtracter 82 and then subtracted from the two-frame delayed signal to determine two-frame difference therebetween. The two-frame difference signal is passed through the band pass filter 83 and then applied to the absolute value circuit 84 whereat the absolute value thereof is determined. The determined absolute value is then converted by the nonlinear converting circuit 85 into a signal 107 indicative of the amount of movement in the C signal. This signal 107 is outputted from the output 89 of the C-signal detecting circuit 7.
The synthesizing circuit 8 is adapted to select and output one of the Y-signal and C-signal movement signals 106 and 107, which is larger than the other movement.
Such a judgement is represented by a control signal 108 in the form of motion coefficient (0.ltoreq.k.ltoreq.1). If a picture is judged to be a complete still picture, the motion coefficient k is equal to zero. If the picture is judged to be a complete motion picture, the motion coefficient k is equal to one.
Generally, if a picture is a still picture, the interframe correlation is utilized to perform the interframe YC separation such that Y and C signals are separated from each other.
As shown in FIG. 13, the interframe YC separating circuit 5 may comprise a one-frame delay circuit 64, an adder 65 and a subtracter 66. V signal 101, inputted to the interframe YC separating circuit 5 at its input 61, is delayed by one frame at the one-frame delay circuit 64 to form a one-frame delay signal which in turn is added to the V signal directly inputted to the adder 65. The resultant one-frame sum provides a YF signal 104 which is outputted from one output 62 in the interframe YC separating circuit 5. At the same time, the subtracter 66 subtracts the YF signal 104 from the V signal 101 directly applied from the input 61 to the subtracter 66 to extract a CF signal 105 which in turn is outputted from the output 63 of the interframe YC separating circuit 5.
In general, if a picture is a motion picture, the infield correlation is utilized to perform the infield YC separation such that the Y and C signals are separated from each other.
As shown in FIG. 14, the infield YC separating circuit 4 may comprise a one-line delay (one horizontal line . . . 1 H delay) circuit 74, an adder 75 and a subtracter 76. V signal 101 inputted to the infield YC separating circuit 4 at its input 71 is delayed by one line at the one-line delay circuit 74 to form a one-line delay signal which in turn is added to the V signal directly inputted to the adder 75. The resultant one-line sum provides a Yf signal 102 which is outputted from one output 72 in the infield YC separating circuit 5. At the same time, the subtracter 76 subtracts the Yf signal 104 from the V signal 101 directly applied from the input 71 to the subtracter 76, to extract a Cf signal 103 which in turn is outputted from the output 73 of the infield YC separating circuit 4.
Since the infield and interframe YC separating circuits 4 and 5 are arranged parallel to each other, the motion adaptive YC separation filter can cause the Y-signal mixing circuit 9 to calculate the following equation using the motion coefficient k synthesized by the synthesizer 8: EQU Y=kYf+(1-k)YF
where Yf is an output Y signal 102 from the infield YC separation and YF is an output Y signal 104 from the interframe YC separation. There is thus obtained a motion adaptive YC separation Y signal 109 which in turn is outputted from the motion adaptive YC separation filter at the output 2.
Similarly, the control signal 108 is utilized to cause the C-signal mixing circuit 10 to calculate the following equation: EQU C=kCf+(1-k)CF
where Cf is an output signal 103 from the infield YC separation and CF is an output signal 105 from the interframe YC separation. There is thus obtained a motion adaptive YC separation C signal 110 which in turn is outputted from the output 3.
The C-signal motion detecting circuit 7 may be arranged as shown in FIG. 15. In this figure, V signal 101 inputted to the circuit 7 at the input 11 is demodulated by a color demodulating circuit 86 into two color difference signals R-Y and B-Y. These color difference signals R-Y and B-Y are then applied to a time division multiplexer 87 in which they are time-division multiplexed at a certain frequency. The output signal from the time division multiplexer 87 is then subjected to subtraction from an output signal from a two-frame delay circuit 81. There is thus obtained a two-frame difference signal.
The two-frame difference signal is passed through LPF 88 wherein a Y-signal component is removed therefrom. The output signal of the LPF 88 is then applied to an absolute value circuit 84 to extract an absolute value therefrom. The absolute value is then applied to a nonlinear converter 85 wherein it is nonlinearly converted into a C-signal motion detection signal 107 which in turn is outputted from the output 89 of the C-signal motion detecting circuit 7.
It will be apparent from the foregoing that Yf and Cf signals from the infield YC separating circuit 4 and YF and CF signals from the interframe YC separating circuit 5 are respectively mixed with each other, based on the amount of movement which is obtained by synthesizing the motion signals from the respective Y-signal and C-signal motion detecting circuits 6 and 7.
Therefore, the filter characteristics for the still picture will be completely different from that for the motion picture. If a picture is switched from a still to a motion picture or vice versa, the resolution is subjected to severe change such that the quality of picture will be remarkably degraded upon processing of the motion picture.